In general, in order to manufacture a full color active matrix organic light emitting diode (hereinafter, abbreviated as ‘AMOLED’), it is necessary to pattern pixels emitting a red light component, a green light component, and a blue light component. For this purpose, a method that deposits R, G, and B color-emitting layers as separated from each other by metal masks has been generally used.
However, the above method has the following two serious problems.
First, small and medium size displays have lower contrast resolution than a TFT-LCD. The reason is that the patterning method that uses metal masks has color mixture problems and a difficulty in manufacturing fine resolution masks, which results in having a limitation to the reduction of the gap between R, G, and B color components.
Second, in the case of a large size TV, the weight of a metal mask is increased, which causes problems in processes. Further, a glass substrate is hung down, which causes a gap between the substrate and the mask.
In order to solve the above problems, a method that separates R, G, and B colors using a color filter after depositing a white OLED has been suggested. The AMOLED that uses the white OLED has an advantage of a simplified patterning method as compared with the patterning method that uses metal masks, but the emitting efficiency is about 30% less than the R, G, and B separation method.
There have been efforts to increase the emitting efficiency by developing new emitting materials and improving device structures. However, there is still a restriction that the emitting efficiency is considerably lower than the R, G, and B separation method.
Accordingly, it is required to seek methods that have the same effect as the increased emitting efficiency using an indirect method. One of the methods is related with the circular polarizer which is used for the increase of the contrast ratio of AMOLED. The light transmittance of the circular polarizer is generally about 45%. In some cases, the transmittance may be increased up to 70% by sacrificing the characteristic of the circular polarizer. The circular polarizer that is generally attached onto a surface of the display may be removed or the light transmittance thereof may be increased for the increase of output brightness.
However, in the case where the circular polarizer is removed or the transmittance of the circular polarizer is improved, the lowering of the contrast ratio of the AMOLED is inevitable.